One of the worst aspects of the hepatitis C virus is its persistence. The virus, which infects an estimated 170 million people worldwide, can hang on for decades after gaining entrance to the body (usually via intravenous drug use or blood transfusion), steadily chewing away at the liver and eventually making a difficult-to-obtain transplant the only chance for survival. The most effective treatment, interferon therapy, works only about half the time and often causes debilitating side effects.
Now, UTMB researchers working with scientists at UT-Southwestern Medical Center in Dallas have discovered a key tactic the hepatitis C virus uses to evade immune defenses. They've also identified a compound that defeats that strategy, quickly restoring immune response in test-tube experiments. And as it turns out, that compound—called a protease inhibitor—already had been under laboratory investigation at UTMB for its ability to slow the progress of hepatitis C in another way, by keeping the virus from making proteins it needs to reproduce itself.
“Our results show a dual effect of this compound, blocking viral replication and allowing the innate immune response of the cell to recover,” says Kui Li, UTMB assistant professor of microbiology and immunology. “This serves as a double whammy against hepatitis C.”
Li, postdoctoral fellow Masanori Ikeda, and Dean of Medicine Stanley Lemon, director of UTMB's NIH Hepatitis C Cooperative Research Center, collaborated on the study with a UT-Southwestern team led by Michael Gale. The journal Science published their results in its April 17, 2003, issue.
In order to make the discovery, the researchers had to overcome a significant handicap: the hepatitis C virus refuses to grow outside of a human or chimpanzee body. To study the virus in test tubes, scientists use “replicons,” strands of hepatitis C genetic material that mimic the replication process but don't produce actual viruses.
When the UT-Southwestern group wanted to investigate how the hepatitis C virus interacts with interferon regulatory factor 3 (IRF-3), which helps mobilize the antiviral defenses of cells, they turned to UTMB's Ikeda and Li, who had developed a highly efficient replicon system. Using the UTMB replicons, the UT-Southwestern team determined that a viral protein known as NS3/4A prevents IRF-3 from doing its job.
“This was an interesting phenomenon, because the NS3/4A complex is the main viral protease,” Li says. “It cuts other proteins expressed by the virus in just the right way so that they can form the machinery needed to amplify the virus' genetic information in infected cells.”
Surprisingly, the viral protease was accomplishing its dirty work by also cutting a normal human protein.“The IRF-3 system is like a burglar alarm, and it goes off when a virus enters the cell, telling the nucleus to activate its defenses,” Lemon says. “The cellular protein that is cut by NS3/4A is like the wire connecting a motion detector to the alarm: the motion detector senses the virus, but its signal can't set off the alarm.”
The results also suggested that something that could inhibit protease activity could also keep the complex from blocking IRF-3 and cellular antiviral action. The UTMB group had a compound that would enable them to check this, a protease inhibitor known as SCH-6 that they were testing to determine how well it blocked hepatitis C virus replication. And when Li treated replicon cells with SCH-6, he found that IRF-3 activity and antiviral response were restored.
The work received considerable attention, garnering mention in a Science Perspectives article and the Highlights section of Nature Reviews Drug Discovery. More important, it galvanized efforts to study protease inhibitors as a treatment for hepatitis C. “These new findings suggest that protease inhibitors will become an important addition to existing interferon treatments for hepatitis C,” Lemon says.
Li intends to pursue this investigation, working as one of three Liver Scholars named by the American Liver Foundation this year. As a Liver Scholar, he will receive a grant of $150,000 over the next three years.